When planes equipped with pontoons are sitting in water, they shake, making the experience less enjoyable and the planes far less aesthetic. I am playing on a singleplayer worls in MC 1.12.2 with just IV and MTS V16 installed.

Fixed with the ground physics update in V19.0.0.

Can confirm this. Really quite annoying.

i believe this is an issue with the way the water handling physics is, and i also notice it jitters/shakes more often on a vehicle that is staying still, aka parkingbraked, or translated from land to water (amphibious stuff). the hope is that this can be fixed with a bit of physics

It's in the works, but since we don't have a consensus on how boat physics should be I'm not too concerned about it right now.